This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to improving servo performance of a disc drive through the generation and use of a continuous, high order position error signal to compensate for head construction and performance variations.
Disc drives are used as primary data storage devices in modern computer systems and networks, due to the efficient and cost-effective manner in which large amounts of computerized data can be stored and retrieved. Disc drives of the present generation have data storage capacities measured in excess of several gigabytes (GB) and can be used alone (as in a typical personal computer configuration) or in multi-drive data storage arrays (as with an internet network server or a mainframe computer).
A typical disc drive comprises a plurality of rigid magnetic storage discs which are journaled about a spindle motor for rotation at a constant high speed. An array of read/write transducing heads are provided to transfer data between tracks of the discs and a host computer in which the disc drive is mounted. The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo system.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly measured and compared to a velocity profile defining a desired velocity trajectory for the seek. Once the head has settled on the destination track, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Both track seeking and track following operations typically require generation of a position error signal (PES) which gives an indication of the radial position of the head with respect to the tracks on the disc. The PES is typically derived from servo data embedded on each recording surface among user data blocks at predetermined intervals.
The head provides the servo data to the servo system which, in turn, generates the PES with a magnitude that is typically equal to zero when the head is positioned over the center of the track (xe2x80x9con trackxe2x80x9d), and is nominally linearly proportional to a relative misposition distance (xe2x80x9coff trackxe2x80x9d) between the head and the center of the track, with a polarity indicative of radial off track direction. To provide stable operation, the transfer function relating PES to actual radial misposition should be constant and independent of distance off track in the presence of variations in head signal amplitude, linear recording bit density, head to media spacing and head skew angle.
Recently, magneto-resistive (MR) heads have supplanted prior use of thin film heads due to the superior magnetic recording properties associated with MR heads. Generally, an MR head includes a magneto-resistive read element characterized as having a baseline direct current (DC) electrical resistance that changes when subjected to magnetic fields of selected orientation. An MR head can detect previously recorded data in response to variations in voltage measured across the MR head when a read bias current of predetermined magnitude is passed through the MR element.
Although MR heads facilitate ever greater levels of magnetic recording densities, there are nevertheless disadvantages associated with such heads when used as position transducers, due to nonlinear readback response with respect to position. This nonlinearity is due to a variety of factors, including MR element bias, sensitivity differences across pole face geometry of the MR elements, and aggravation of the inherent nonlinearity of end fringing fields, as the width of a typical MR element readback gap is typically appreciably less than the width of the tracks on the associated disc.
Historically, PES nonlinearity primarily affected track seeking performance, producing track arrival over-shoot or under-shoot which prolonged arrival settling time. PES nonlinearity did not so adversely affect track following performance (i.e., did not induce significant track misregistration, or TMR), because the track center for reading and writing, defined by servo bursts of the servo information (typically referred to as A, B, C and D bursts) nominally coincided with a null response of positioning burst signals Axe2x88x92B=0.
With MR heads, however, PES nonlinearity also affects TMR control during track following because the MR element is physically separated from the write element of the head by a distance governed by the design of the head; thus, inaccuracies can arise as a result of head skew (with respect to the disc) and head fabrication tolerances. Because the write gap and the read gap are at slightly different radial positions, the PES must be operated at a position Axe2x89xa0B to properly center the write gap over the track. Further, nonuniformity in readback magnetic field sensitivity generally produces a positional shift in null response of bursts Axe2x88x92B=0. Unless properly characterized and compensated, the nonlinearity will produce a discrepancy from desired positioning and can introduce instabilities into the servo system.
PES nonlinearity due to the use of MR heads can not only therefore adversely affect seek performance, but track following performance as well. Such effects can further serve as a limit on achievable track densities (and hence data storage capacities), due to the TMR budget necessary to minimize interference with (such as overwriting of) adjacent tracks.
Accordingly, there remains a continual need in the art for improvements whereby disc drive performance can be optimized through minimizing the effects of head position nonlinearities.
The present invention provides an apparatus and method for improving disc drive servo control performance.
In accordance with preferred embodiments, a rotatable disc has first, second, third and fourth radially staggered servo burst patterns which define a plurality of concentric tracks. An actuator assembly supports a transducing head adjacent the tracks.
A servo circuit combines respective at least third order polynomial functions of burst signals from the first and second, and from the third and fourth, burst patterns to generate a continuous function position error signal (PES). The PES preferably has normalized absolute magnitudes of 0 when the head is positioned at track centers, 0.5 at track boundaries, and 0.25 at quarter-track locations, and with substantially linear slope therebetween. The servo circuit applies current to an actuator motor to position of the head relative to a selected track in relation to the magnitude of the PES.
An advantage associated with the present invention is the potentially wider range of head width, sensitivity and symmetry variations that can be accommodated using the present invention as embodied herein. A significant economic benefit can inure due to reduced manufacturing costs through the relaxation of head tolerances, improvements in manufacturing yields and reductions in manufacturing costs.
Moreover, the present invention can be further utilized to provide feedback to a supplier of the heads, resulting in improved head manufacturing process control.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.